There is known, for example, from the document FR-A-2 622 055, an antenna used in such a system. It has a directional diagram with two main lobes, one of them being asymmetrical from the other relative to a plane that is orthogonal to its main plane. When it is mounted on a vehicle following the Janus configuration, one of the two main lobes is forward slanted and the other one is rearward slanted; the plane of symmetry between these two lobes being orthogonal to the direction of forward motion of the vehicle.
The antenna described in the above document is constituted of a plurality of identical linear sub-networks, that are parallel and symmetrical, the Centers of which are lined up along a line perpendicular to their longitudinal direction, and are fed in phase. Each sub-network is made up of a plurality of radiating elements that radiate from the field in opposition of phase from one element to the next. The step between each element is equal to a wave length guided on the substratum of the circuit on which they are printed and corresponding to the frequency of operation of the antenna.
Advantageously, each radiating element is alternately placed on one side or the other of a secondary feed line fed at the center of symmetry of the sub-networks.
Moreover, each radiating element is made up of a conductor square surface the side of which is approximately equal to the half-length of (the) guided wave. A corner is galvanically connected to the secondary feed line, and the diagonal passing through that point of galvanic contact is perpendicular to the longitudinal direction of the sub-network.
In FIG. 1 there is shown a printed circuit board or plate antenna A that sends two beams F1, F2 through which there passes a plane H located in the axis of the antenna and orthogonal to its surface. These beams are symmetrical relative to a plane E that is orthogonal to plane H and to the surface of antenna A.
An antenna such as just described presents drawbacks. Among the latter, there may be mentioned the fact that it has a radiating power, in plane E, that is relatively high relative to the one sent into plane H. That phenomenon causes difficulties for the treatment of the signal delivered by the antenna, so that, in some cases, errors in measurement may occur.
A second drawback results from its set structure that renders difficult its application to measurement systems with a Janus configuration, that offers special geometrical characteristics. For example, the angle that each lobe forms relative to a perpendicular to the main plane of the antenna, called de-pointing (coil unwinding) angle, is 41.8 degrees and its value can be modified only by changing the material of the substratum, that is to say by modifying its dielectric constant.